Parakeets, aka budgies, make for sweet, usually peaceful companions, but like most pets they can become aggressive under the right conditions. Though they rarely fight in the wild, caged parakeets may battle for space or dominance. Sometimes this aggression works itself out, but not always.
Hormonal Aggression
Sudden aggressive changes in your parakeet's moods are generally nothing to be alarmed about. Often it means that your bird's estrogen or testosterone levels are increasing, causing physical and behavioral changes. This usually happens when parakeets reach sexual maturity or are looking to mate. Aggressive behaviors toward other birds and people, such as hissing or shunning your finger, are not uncommon in hormonal parakeets.
Worse in Females
Female parakeets are the more aggressive gender—particularly toward other females. If you have a lone female, do not add another to the cage. The two are likely to fight. Conflicts between two males, on the other hand, are rare. A lone female who becomes aggressive is best left alone for three to six weeks. Females generally accept a male, but not always. In such cases, replace the male with a different one.
Signs of Aggression
Parakeets are generally obvious with their aggressive behaviors. Open-beak whistling or screaming usually means your bird is about to bite. Raised wings and a lowered head often indicate a threat of attack as well. Rapid pupil expansion or contraction usually means she's scared. So do squatting, head bending and rocking. Never try to touch a frightened bird, she will likely bite or attack you. Give her time and space to cool down.
Coping with Aggression
It may get frustrating to deal with an angry, screeching or aggressive bird, but it is vital to remain calm. Do not get angry with your parakeet for being aggressive, and never hit or grab her. To help ease aggression in breeding parakeets, make sure the cage is large enough for all the birds to have some personal space. If you're colony breeding, make sure no single birds are in among the couples, to reduce the chances of an attack.